你可以在这里看到他们的完整信息：https://developers.google.com/speed/public-dns/docs/security#rate_limit

Google Public DNS implements two kinds of rate control:

• Rate control of outgoing requests to other nameservers. To protect other DNS nameservers against DoS attacks that could be launched from
  our resolver servers,Google Public DNS enforces per-nameserver QPS
  limits on outgoing requests from each serving cluster.

• Rate control of outgoing responses to clients. To protect any other systems against amplification and traditional distributed DoS (botnet)
  attacks that could be launched from our resolver servers,Google
  Public DNS performs two types of rate limiting on client queries: To
  protect against traditional volume-based attacks,each server
  imposes per-client-IP QPS and average bandwidth limits.
  To guard against amplification attacks,in which large responses to small queries are exploited,each server enforces a
  per-client-IP maximum average amplification factor. The average
  amplification factor is a configurable ratio of response-to-query
  size,determined from historical traffic patterns observed in our
  server logs.

  If queries from a specific source IP address exceed the maximum QPS,or exceed the average bandwidth or amplification limit consistently (the occasional large response will pass),we return (small) error responses or no response at all.